Processes are known in the art for the purification of waste liquors in the production of phenol-formaldehyde resins by isolating phenol by way of extraction with the aid of solvents.
It is known to isolate phenol in the form of an oily film by adding ammonium salts and a strong acid to waste liquors.
Processes for the purification of waste liquors from phenol using adsorbents have been disclosed.
It is known to purify waste liquors from phenol by boiling, with dephlegmation of steam. With autoclaving and high pressure and temperature, the degree of phenol extraction reaches 99.4%.
Processes are known in the art for the purification of waste liquors from aldehyde.
All the known processes involve purification of waste liquors only from either phenol, or formaldehyde or from a mixture thereof, yet the known processes cannot provide for the purification from other noxious substances present in waste liquors. In addition, the degree of purification is inadequate.
Waste liquors in the production of phenol-formaldehyde resins also contain methanol, a catalyst and formic acid which are introduced with the feedstock for the production of resin, as well as products formed during the preparation of the resin.
It is known that waste liquors in the production of phenol-formaldehyde resins contain a considerable excess of formaldehyde and much water.
For recovery of phenol and formaldehyde from waste liquors, they should be subjected to a secondary polycondensation, excess water should be removed, and a useful product obtained. However, in order to obtain a product (resin) suitable for further utilization, the molar ratio of phenol to formaldehyde should be close to the equimolar ratio, and water should be removed under mild conditions.
A process is known in the art for the purification of waste liquors in the production of phenol-formaldehyde resins, wherein phenol is added to waste liquors containing phenol and formaldehyde in the molar ratio of 1:4.18 in an amount such that the phenol-to-formaldehyde ratio is from 1:1.02 to 1:1.12. The purification process is conducted in two stages. At the first stage, the mixture is heated at 80.degree.-85.degree. C. for at least 30 hours, and then, at the second stage, water is removed at a temperature not exceeding 116.degree. C. to prevent the formation of resite.
The resultant liquid resin may be used in the production of ordinary novolac resin.
The disadvantage of the above-described process resides in the high consumption rate of costly phenol (109 kg per ton of waste liquor), long processing time (the first stage taking from 30 hours to several days) and, hence, high power inputs. The process is very complicated because it involves stringent maintenance of preset temperature conditions at all stages.
A further process known in the art is the purification of waste liquors in the production of phenol-formaldehyde resins, wherein phenol, formaldehyde and an acid are added to waste liquors containing 2.88% of phenol and 1.77% of formaldehyde to obtain a phenol-to-formaldehyde molar ratio of 1:1; then an adsorbent in the form of saw-dust is added, and the mixture is heated. Polycondensation is conducted during three hours. The resultant product is isolated from the liquid phase and utilized.
The liquid phase is passed through saw-dust during 1.6 hours, whereafter it contains 0.15% of phenol and 0.89% of formaldehyde.
The disadvantage of this process resides in the high content of phenol and formaldehyde and presence of methanol in the resultant waste liquors, the need for the addition of costly phenol, formaldehyde and acid, and subsequent filtering of waste liquors after polycondensation, as well as in the processing at low pH (0.47) which leads to intensification of hydrolysis of wood.